1. Field of the Invention
The present invention relates to a method and apparatus for controlling the combustion in a gas turbine and, more specifically, a method and apparatus for controlling the combustion in the presence of gaseous fuels having significantly different Wobbe indexes and variable with time.
2. Description of the Related Art
The use of gas turbines, normally consisting of a multiphase compressor, in which the air sucked from the outside is compressed, a combustion chamber, in which the combustion takes place of gaseous fuel added to the compressed air, and a turbine or expander, in which the gases coming from the combustion chamber are expanded, is known for the production of electric energy. The turbine is then capable of generating mechanical energy which can be exploited for activating operating machines or for feeding electric generators.
There are various types of gases or gaseous mixtures which can be used as fuel in a gas turbine. The calorific value and consequently the energy which each gas or gaseous mixture is capable of developing inside a same combustion chamber can therefore vary considerably. The temperature of the gas, or gaseous mixture, used as fuel is also capable of significantly influencing the performances of a gas turbine.
The use of a parameter called “Wobbe index” is known for measuring the heat which is produced by a gas, or mixture of gases, when it is burnt at a constant supply pressure. It is equivalent to the ratio between the upper calorific value (or lower) of the gas and the square root of the relative density of the gas, measured with respect to the air, on the basis of the following formula:
      I    w    =      PC                            T          G                *                  G          s                    wherein:IW=Wobbe index;PC=calorific value (upper or lower) of the gas;TG=temperature of the gas;GS=relative density (or specific gravity) of the gas.
The heat produced by a gas is therefore directly proportional to the Wobbe index, as well as to the area of the nozzle of the turbine from which the gas used as fuel exits. The Wobbe index is consequently an important parameter for determining the interexchangeability of different gases which can be used as fuels in a certain gas turbine.
Most low-emission gas turbines are currently configured for operating with gaseous fuels having small differences in terms of Wobbe index. The use of different fuels, i.e. having high variations in the Wobbe index, in the same turbine can in fact lead to irregular combustion processes which can cause malfunctioning of the turbine itself, reduction in the operative cycle of the components of the combustion system and possible interruptions in the generation of energy.